Abstract
This paper gives case histories of several materials of construction that have been used for bottom scrapers in continuous digesters. The predominant mechanisms of deterioration are erosion-corrosion thinning and stress corrosion cracking (SCC). In some cases, carbon steel bottom scraper arms have experienced rapid erosion-corrosion thinning at rates approaching 1 inch per year (25 mm/y). Both type 304L austenitic stainless steel and type 2205 duplex stainless steel have both experienced extensive SCC in bottom scraper service. Sulfur species may be the SCC agent in the bottom scraper environment where the alkaline pulping liquor is diluted with filtrate. In some cases, cracking initiated by SCC has propagated under fatigue and has caused failures of stainless steel arms. There has been no practical material of construction that has resisted both erosion-corrosion and cracking in bottom scraper service. Nickel-base alloys resist erosion-corrosion and may be immune to SCC but may not be practical due to their high cost. One possible compromise is to construct new bottom scrapers using carbon steel where the leading edges of the arms are protected using nickel-base alloy weld overlay.
